The present invention relates to wireless power transfer systems, and more particularly to systems and methods for providing communications in a wireless power transfer system.
Many conventional wireless power supply systems rely on inductive power transfer to convey electrical power without wires. A typical inductive power transfer system includes an inductive power supply that uses a primary coil to wirelessly transfer energy in the form of a varying electromagnetic field and a remote device that uses a secondary coil to convert the energy in the electromagnetic field into electrical power. Recognizing the potential benefits, some developers have focused on producing wireless power supply systems with adaptive control systems capable of adapting to maximize efficiency and provide appropriate operation to a variety of different types of devices under a wide range of circumstances. Adaptive control systems may vary operating parameters such as resonant frequency, operating frequency, rail voltage or duty cycle, to supply the appropriate amount of power and to adjust to various operating conditions. For example, it may be desirable to vary the operating parameters of the wireless power supply based on the number of electronic device(s), the general power requirements of the electronic device(s) and the instantaneous power needs of the electronic device(s). As another example, the distance, location and orientation of the electronic device(s) with respect to the primary coil may affect the efficiency of the power transfer, and variations in operating parameters may be used to optimize operation. In a further example, the presence of parasitic metal in range of the wireless power supply may affect performance or present other undesirable issues. The adaptive control system may respond to the presence of parasitic metal by adjusting operating parameters or shutting down the power supply. In addition to these examples, those skilled in the field will recognize additional benefits from the use of an adaptive control system.
To provide improved efficiency and other benefits, it is not uncommon for conventional wireless power supply systems to incorporate a communication system that allows the remote device to communicate with the power supply. In some cases, the communication system allows one-way communication from the remote device to the power supply. In other cases, the system provides bi-directional communications that allow communication to flow in both directions. For example, the wireless power supply and the remote device may perform a handshake or otherwise communicate to establish that the remote device is compatible with the wireless power supply. The remote device may also communicate its general power requirements, as well as information representative of the amount of power it is receiving from the wireless power supply. This information may allow the wireless power supply to adjust its operating parameters to supply the appropriate amount of power at optimum efficiency. These and other benefits may result from the existence of a communication channel from the remote device to the wireless power supply.
An efficient and effective method for providing communication in a wireless power supply that transfers power using an inductive field is to overlay the communications on the inductive field. This allows communication without the need to add a separate wireless communication link. One common method for embedding communications in the inductive field is referred to as “backscatter modulation.” Backscatter modulation relies on the principle that the impedance of the remote device is conveyed back to the power supply through reflected impedance. With backscatter modulation, the impedance of the remote device is selectively varied to create a data stream (e.g. a bit stream) that is conveyed to power supply by reflected impedance. For example, the impedance may be modulated by selectively applying a load resistor to the secondary circuit. The power supply monitors a characteristic of the power in the tank circuit that is impacted by the reflected impedance. For example, the power supply may monitor the current in the tank circuit for fluctuations that represent a data stream.
A variety of schemes have been developed for encoding data that is transmitted on an inductive field using backscatter modulation. One common approach is bi-phase modulation. Bi-phase modulation uses a scheme in which the signal varies from high to low at every clock pulse. To encode a “1,” the modulator adds an additional transition during the time period associated with that bit. To encode a “0,” the clock pulse transition is the only transition to occur during the time period associated with that bit.
Wireless power communications can be disrupted if the device being powered presents a noisy load. For example, changes in the amount of power consumed in a device may change the impedance of the remote device. These changes in impedance may create the appearance of data when none exists or they may corrupt legitimate data. The power supply can be especially susceptible to noise that occurs at that same frequency as the data communications. For example, it is possible that load fluctuations occurring while data is being transmitted will mask the legitimate data. As another example, if occurring in the same frequency range as the data communications, it is possible that a random pattern in the noise will be misinterpreted as the preamble or start bits in a legitimate communication signal. If this occurs, the power supply may think it is receiving legitimate data and attempt to extract data, for example, in the form of a data packet, following the faux preamble. Although the power supply should eventually determine that the data packet is not legitimate, the power supply may be occupied with the illegitimate packet, which would delay its ability to recognize legitimate data.
Further, in some applications, the remote device is configured to send “keep-alive” signals to the wireless power supply. The keep-alive signal may, for example, tell the wireless power supply that a compatible remote device that needs power is present. If noise prevents a consecutive number of keep-alive signals from being recognized by the wireless power supply, the supply of power to the device may be discontinued. This can be particularly problematic when the remote device battery is depleted.